It is typical in rotating shaft devices, and especially in impeller driven pumps, for pressure differences to be developed within the mechanism that result in axially directed forces, generally referred to as “thrust,” being applied to the rotating shaft. For example, in a centrifugal pump, the impeller (or each impeller) will tend to produce some amount of thrust because of different pressures and different geometries on the two sides of the impeller.
In some cases, these axial thrust forces are opposed and absorbed by the bearings that support the rotating shaft. However, it can be undesirable to require that the bearings absorb all of the thrust that is generated by the impellers. For example, in a high pressure multistage pump the net thrust that is generated may cause unacceptable wear to the bearings unless it is compensated in some manner. Accordingly, it is often desirable to include a mechanism within a rotating shaft device that will compensate for thrust effects by generating an offsetting thrust, thereby reducing or eliminating the thrust compensating load that is placed on the bearings
Thrust that arises in a multi-stage rotary pump can sometimes be offset, for example in axial split pumps, by including an even number of stages, and by orienting the impellors in opposite directions, such that the thrust developed by one half of the pump stages is offset by an approximately equal and opposite thrust developed by the other half of the pump stages. However, it is not always practical to balance axial thrust by using opposed impellers, especially for pumps such as barrel pumps that operate at high pressures. Furthermore, even for pumps with opposed impellors the innermost impellor stages will tend to create a net axial thrust that depends on the pressure within the pump.
Another approach that is used for thrust compensation is to include a balancing “disk.” A simplified example is presented in the cross-sectional illustration of FIG. 1, in which an impellor 100 is fixed to a rotating shaft 102. In this example, process fluid that leaks past the impellor 100 is collected behind the impellor 100 in a leakage chamber 104 formed between the shaft 102 and the pump housing 106. One end of the leakage chamber 104 is bounded by a thrust-balancing “disk” 108, which is fixed to the shaft 100.
The balancing disk 108 is configured such that a narrow, axial gap 110 is formed between the outer perimeter of the disk 108 and the pump housing 106. Leakage fluid is able to flow through this “pressure relief” gap 110 at a limited rate into a collection chamber 112 which is in fluid communication with the pump inlet. According to this configuration, the fluid pressure in the collection chamber 112 is approximately equal to the inlet pressure, while the fluid pressure in the leakage chamber 104 is higher than the inlet pressure. As a result, a compensating thrust 116 is applied to the balancing disk 108 that is in opposition to the axial thrust 114 generated by the impellor 100.
If the compensating thrust 116 is less than the impellor thrust 114, the rotating shaft 100 is axially shifted to the right, causing the pressure relief gap 110 to be narrowed, and raising the pressure in the leakage chamber 104, thereby increasing the balancing thrust 116. Conversely, if the balancing thrust 116 is greater than the impellor thrust 114, then the shaft 100 is axially shifted to the left and the pressure relief gap 110 is enlarged, thereby reducing the pressure in the leakage chamber 104. The result is a self-regulating effect that can maintain the axial thrust at a very low level, which can approach zero net thrust, because the compensating thrust reacts directly to the axial shifting of the rotating shaft 100, which is caused by the residual axial thrust.
It is clear from FIG. 1 that the radial pressure relief gap 110 is critical to the thrust compensation. Unfortunately, for some pump designs there can be physical contact between the balancing disk 108 and the housing 106, for example during pump startup and/or due to unexpected fluctuations in pump speed. Accordingly a balancing disk is not always a suitable approach for axial thrust compensation.
Another approach that is sometimes used for thrust compensation, for example when a wide range of operating speeds is anticipated and/or where there may be transient fluctuations in the pump speed, is to include a balancing “drum.” A simplified example is illustrated in FIG. 2.
In the example of FIG. 2, the leakage chamber 104 behind the impellor 100 is terminated at one end by a so-called balancing “drum” 200, which differs from the balancing disk 108 of FIG. 1 mainly in that it is separated from the housing 106 by a radial gap 202 instead of an axial gap 110. In the example of FIG. 2, a compensating thrust 116 is created essentially by the same mechanism as for the balancing disk 108 of FIG. 1. The primary difference is that the gap 202 does not vary in size as a function of axial shaft position, so that there is no “self-regulation” of the thrust compensation. Instead, the fluid pressure in the leakage chamber 104 tends to remain at a fixed percentage of the impellor outlet pressure. The advantage of the balancing drum approach is that there is little or no danger of contact and wear between the drum 200 and the housing 106. The disadvantage is that a balancing drum does not respond directly to changes in axial position of the shaft, and as a result the residual thrust 114 will tend to vary over a wider range than for a balancing disk, especially if the pump is operated at varying speeds. Accordingly, the bearings can be required to absorb greater residual thrusts than in the case of a balancing disk.
What is needed, therefore, is an axial thrust balancing mechanism that provides a self-regulating and potentially near-complete balancing of the axial thrust in a rotating shaft system, while avoiding any possibility of contact and wear between the balancing mechanism and the apparatus housing.